1. Field of the Invention
The present invention relates to information recording and reproduction apparatuses for recording and reproducing information using a laser beam. More particularly, the present invention relates to an information recording and reproduction apparatus that can reproduce at high density information from an information recording medium such as a magneto-optical recording medium and a phase transition recording medium by emitting a pulsed laser beam in reproduction.
2. Description of the Background Art
A magneto-optical recording medium is noteworthy of its rewritable ability, large storage capacity, and high reliability. It is already available for practical usage as the memory and the like for computers. However, the recording and reproducing technique of information at higher density is required in accordance with increase in the amount of information to be recorded and reduction in the size of the recording and reproduction apparatus.
The technique for recording and reproducing information at higher density is divided into the technique at the recording and reproduction apparatus end and the technique of the recording medium end.
The former includes, in addition to the method of rendering the wavelength of a laser beam shorter, the so-called optical superresolution method. This method achieves a focused spot that exceeds the diffraction limit of a laser beam by inserting a light blocking object in the light path of the laser beam. This optical superresolution method is disclosed in "High Density Optical Recording by Superresolution", Japanese Journal of Applied Physics, Vol. 28, Supplement 28-3, pp. 197-200, 1989 by Y. Yamanaka et al., for example.
The latter technique includes, in addition to the method of narrowing the pitch of the recording track of the medium, the method of improving the reproducing resolution using a magnetic multilayer film. This art of improving reproduction resolution using a magnetic multilayer film includes the step of providing a magneto-optical recording medium with a magnetic multilayer film having a recording layer and a reproduction layer. By taking advantage that the temperature distribution within the laser spot exhibits a Gaussian distribution that takes the maximum value around the center thereof, the magnetized state of the recording layer is selectively transferred to the reproduction layer by virtue of the exchange-couple force when being irradiated with a laser beam for reproduction. The magnetized state of that reproduction layer is read out at high density with a laser beam of a light power that is lower than that of recording. This technique is disclosed in, for example, "Recent Progress in Magnetically Induced Superresolution", Proceedings of Magneto-Optical Recording International Symposium '96, Journal of Magnetics Society of Japan, Vol. 20, Supplement No. S1, pp. 7-12, 1996 by M. Kaneko et al.
Conventional reproduction of information from such a magneto-optical recording medium is effected by continuously emitting a laser beam on the recording surface of a medium. However, there was a problem that reproduction at high density cannot be implemented when the laser beam for reproduction is emitted continuously.
This problem will be specifically described hereinafter. By using the optical superresolution method, the diameter of the laser beam for reproduction can be reduced up to approximately 0.78 .mu.m at the present stage. The shortest domain length that can be reproduced with this beam diameter is 0.4 .mu.m beam which corresponds to approximately half the diameter. At the current stage, the shortest domain length that can be recorded has become as small as approximately 0.15 .mu.m. There was a problem that, even if the optical superresolution method is used, information recorded at a domain smaller than 0.40 .mu.m cannot be reproduced. The reproduction technique cannot follow the currently available high density recording.
This problem is due to the fact that the laser beam is continuously emitted at the time of reproduction. This continuous emission of a laser beam will result in a greater laser spot for reproduction on the signal surface due to the fact that heat diffusion is generated on the signal surface of the medium and that the laser spot on the signal surface is extended by the relative movement between the medium and the laser beam. As a result, information recorded in a small domain cannot be reproduced.
This continuous emission of a laser beam also induces the problem that the laser output is gradually reduced since the semiconductor laser which is the light source is constantly turned on during reproduction. The lifetime of the semiconductor laser is also shortened and power consumption is increased by this continuous usage.